What is IDEA (International Data Encryption Algorithm)?

IDEA is a 64-bit block cipher with a 128-bit key and 8.5 rounds, notable for mixing three algebraic operations from different groups.

Parameters:

Unique design: Combines three operations from different algebraic groups:

• XOR (addition in $\mathrm{GF}(2^{16})$)
• Addition modulo $2^{16}$
• Multiplication modulo $2^{16} + 1$

History and status:

• Designed by Xuejia Lai and James Massey (ETH Zurich) in 1991
• Used in PGP (Pretty Good Privacy) email encryption
• Was patented until 2012
• Considered secure but largely replaced by AES
• Notable as a Swiss-designed cipher (ETH Zurich) — one of the first strong, publicly available block ciphers

Go deeper:

• IDEA (Wikipedia) — the three-group mixing design and its PGP heritage.